Differences Between Audit and History Sets
Summarize
Summary of Differences Between Audit and History Sets
ServiceNow stores historical data about record changes in three related tables: Audit [sysaudit], History Set [syshistoryset], and History [syshistoryline]. While these tables contain overlapping data, they serve distinct purposes and manage data differently to optimize performance and usability.
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Audit [sysaudit] Table
The Audit table retains a comprehensive, permanent record of all changes made to audited records. This full history allows administrators to track any changes indefinitely. However, as audit data accumulates over time, directly querying this large table becomes inefficient for retrieving historical information.
History Set [syshistoryset] and History [syshistoryline] Tables
The History Set table identifies specific records from audited tables that have historical data, while the History table stores the detailed changes to field values for those records. These tables contain a recent, limited subset of historical information, generated automatically from the Audit table when a user creates a record or requests its history.
History Sets include not only audit data but also information present at record creation, including journal field entries. Entries created before the record creation are included with the same timestamp and creator details as the record itself.
Data Management and Retention
- The system automatically cleans History Set records that have not been updated for 30 days.
- History tables use rotation, cycling through four History tables every seven days. Because one table is cleared during each rotation, the maximum retention period for History data is 21 days.
- If historical information is needed beyond this retention, it can be regenerated from the Audit records.
User Experience
Users access historical data through the same interface regardless of whether the data comes from History Sets or Audit records. However, the system optimizes performance by using History Set data for recent changes and Audit data as the comprehensive historical source.
The Audit [sys_audit], History Sets [sys_history_set], and History [sys_history_line] tables store the same data, but they serve different purposes and manage data differently.
Audit [sys_audit] table
The Audit [sys_audit] table is where the system stores historical information for all records. These records are intended to be kept forever so that administrators can always track the history of audited records. As the number of auditing records grows over time, it becomes more inefficient to directly query the Audit table for historical information. It is much more efficient to run queries only on the smaller subset records you actually want to view historical information for.
History Set [sys_history_set] table
- The system automatically generates History Set and History records as needed from the Audit table when a user either creates a record or requests its history.
- Rather than containing a complete history of all changes in the system, History Set and History records only contain a recent subset of historical information for records where users have created or requested such information.
- In addition to audit data, history sets also include the information that is set during record insert, including journal field entries. Journal field entries you create before creating a record are handled in the same manner as journal entries created at the time of record creation. These journal entries appear in history sets with the same creation time and created by data as the associated record itself.
The system limits History Set and History records by:
- Having the table cleaner delete History Set records that have not been updated in 30 days.
- Using table rotation to rotate between four History tables every seven days. Because the platform truncates the leading table when rotation occurs, the maximum available retention period is 21 days, not 28. One of the four tables is always in the process of being cleared in preparation for the next rotation.
Should someone need historical information again at a later date, the system can regenerate it from auditing source records.
After the system generates History Set records, the context menu choice History uses the History Set rather than Audit records. From the user's perspective, the same historical data is available in the same user interface, but the way the information is stored is different.